Loving texture paint - How can I make a square 'pixel' brush?
You can use an image rendered by setting up a plane with a shadeless material, looking through an orthographic camera.

Then you can set that resulting image up as a brush mask, and the key is to set it up to calculate alpha and to clip, not to repeat. Here I set up the spacing of the brush to be 40%, and the mapping of the brush mask texture at 200% to get the right amount of brush radius. Also, I set it to use the full hard brush curve so that it would fill well.

I did also think maybe you could just user the image I have made so here you go.